body {
  overflow: visible;
  height: 2000px;
}

.wrapper {
  
  /* overflow: hidden; */
}

.flowBox {
  position: fixed;
  top: 20px;
  left: 0;
  right: 0;
  bottom: 0;
  background-color: #d30000;
  opacity: .7;
  mix-blend-mode: multiply;
}
div.nav {
  height: 100px;
  border: solid deepskyblue;
  margin-top: 50px;
}
nav {
  position: sticky;
  top: 20px;
  background: lightskyblue;
}